1. Field of the Invention
The present invention is generally related to ferroelectric random access memory (FeRAM), and more specifically to method of processing dielectric layer in FeRAM and structure of the like.
2. Description of Related Art
Computer memories are generally categorized into non-volatile and volatile memories. Volatile memories such as DRAMs and SRAMs are usually much faster and more robust but they suffer from the ability to retain information when power is removed. On the other hand, non-volatile such as EEPROM and flash RAM are retain data when power is removed but they are usually slower and have a limited life span.
In the recent years, with the advance of the film formation technique, researches have been extensively made on nonvolatile semiconductor memories using ferroelectric thin films. This nonvolatile semiconductor memory allows high speed rewriting by making use of high-speed polarization reversal and its residual polarization of a ferroelectric thin film. This nonvolatile semiconductor memories including ferroelectric thin films, which are being studied at present, are classified into a type of memory by detecting the amount of electric charges stored in a ferroelectric capacitor portion and a type of detecting a change in resistance of a semiconductor due to spontaneous polarization of a ferroelectric material. The semiconductor memory cell, to which the present invention is applied, is the former type.
Despite the superior performance and virtually unlimited read/write ability of FeRAM, the fabrication method of FeRAM is still not mature enough for mass production. The process of fabricating FeRAM generally includes various heat-treatment steps performed at a hydrogen gas atmosphere at temperatures in a range of 280.degree. to 450.degree. C. In such a heat-treatment, hydrogen gas may permeate an upper electrode. Furthermore hydrogen from the neighboring layers will also serve to damage the FeRAM structure. The penetration of hydrogen greatly affects the performance and durability of the FeRAM. As a result, a method and structure that can effectively block hydrogen at a low cost would be beneficial.
The present invention provides a ferroelectric structure of a FeRAM that can effectively prevent the penetration of hydrogen by using gradient dielectric layers. A ferroelectric capacitor comprising a first electrode, a second electrode, and a ferroelectric material is protected by two gradient dielectric layers on the top and the bottom. The first electrode and the first gradient dielectric layer are separated by a barrier layer. The gradient dielectric layers have a changing refraction index from their bottom side to their top side. The first dielectric layer which is located below the ferroelectric capacitor has an increasing refraction index from the bottom side to the top side, whereas the second dielectric layer which is located over the ferroelectric capacitor and covering the entire FeRAM has a decreasing refraction index from the bottom side to the top side.
The material of the first and second gradient dielectric layer is SiO2. These layers are deposited by plasma enhanced chemical vapor deposition. The refraction index is changed by adjusting the ratio and deposition power of the SiH4 and N2O mixture during the deposition. The result is a linear gradient refraction index from one side to the other side of the first and second dielectric layer. These gradient dielectric layers can effectively prevent hydrogen from damaging the FeRAM.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention is claimed.